Transformer Fusing

[fifty60]

Table 450.3(B) states that I can raise the fuse on a transformer that has less than 9A primary current from 167% to 250% if the secondary is fused at 125% of the secondary current.

If the transformer has is center tapped (120V/240V) and there are loads across the 120V and across the 240V, how is the secondary fuse size calculated?

Am I supposed to use the secondary at 120V or the secondary at 240V or do I have to consider both? I know if there are multiple fuses then the sum is considered in the 125% calculation, but I do not know how to handle center tapped transformers with multiple secondary voltages.

 

[don_resqcapt19]

You use the 240 volts to find the secondary current.

 

[fifty60]

Thanks. Do I have to add the fuses on the 120V tap to the fuses across the 240V, or do I ignore the 120V fuses on the secondary?

 

[Smart $]

Thanks. Do I have to add the fuses on the 120V tap to the fuses across the 240V, or do I ignore the 120V fuses on the secondary?

The fuses go in series with the line conductors L1 and L2. The center tap is for the neutral conductor, required to be grounded in most cases, and as such is not fused.

To determine max' fuse rating simply multiply kVA rating by 1000, divide by 240V, then multiply by 125%.

Typically the secondary fusing also serves as load and load conductor protection. So the max size must equal or exceed the minimum required size for the load and circuit conductors on the load side (note the load should not exceed the kVA rating for any substantial amount of time).

 

[Smart $]

PS: You are not limited to a single fuse each for L1 and L2. You can fuse your loads individually, or otherwise, up to six fuses per line. However, the sum total of fuse ratings per line cannot exceed the max' rating as determine by the method in my earlier post. Also see the requirements for secondary [tap] conductors of 240.21(C).

 

[fifty60]

Interesting. So, for example, if I have a center tapped (120/240) 1KVA transformer I would divide 1000/240 to get 4.16A and multiply that by 1.25 to get 5.2.

I would have to use a 6A fuse or smaller (6A being the rounded up standard size) even if I were only using the 120V across the secondary?

I suppose this would only be true if I were grounding the center tap to derive the 120. If I connected the secondary so that it produces 120V across L1 and L4 without a center tap, then I could use 1000/120 =9A to derive my max fuse size.

I can use whatever fuse sizes I want on the secondary as long as primary current is less than 9A and the primary is fused at less than 167% of primary current?

Please correct me if any of the above is wrong.

 

[don_resqcapt19]

...
I can use whatever fuse sizes I want on the secondary as long as primary current is less than 9A and the primary is fused at less than 167% of primary current? ...

As long as the secondary conductors have an ampacity equal to or greater than that of the secondary fuses.

If you wire the transformer for only a 120 volt output, then you do not need any secondary fusing. See 240.21(C)(1)

 

[Smart $]

... If I connected the secondary so that it produces 120V across L1 and L4 without a center tap...

It'd still be L1 and L2... or just L and N when grounded, assuming we're talking about a control transfomer in a piece of machinery... though many will disagree from the NEC perspective. The secondary configuration is stated by terminals, e.g. X1-X3, X2-X4, but not certain of any convention.

 

[fifty60]

If you have a 480V transformer, and there are not any taps provided for 380V, then you automatically have to derate the transformer KVA by 380/480 = .79%?

The only time you would not derate the transformer KVA is if the transformer actually had 380V and 480V taps right? They could never use the same taps and have the same KVA?

All of the derating above would have to be done before you could calculate the fuse size?

 

[Smart $]

If you have a 480V transformer, and there are not any taps provided for 380V, then you automatically have to derate the transformer KVA by 380/480 = .79%?

The only time you would not derate the transformer KVA is if the transformer actually had 380V and 480V taps right? They could never use the same taps and have the same KVA?

All of the derating above would have to be done before you could calculate the fuse size?

Not really. Yes, the transformer kVA rating would be diminished, but the rated primary and secondary current would be the same... and the fuse sizing is based on rated current.

But you'd not have a 120/240V secondary either...???

 

[fifty60]

If the KVA is Derated then it does not make much sense to use that KVA as the KVA for the fuse sizing. Yes, secondary is 95V. So using 3000 would have 31.57 secondary Amps. If I derate for the 380V I get 2.375KVA with secondary amps of 25A.

31.75 * 1.25 = 40A fuse

25 * 1.25 = 35A fuse.

Relatively big difference (5 * 95 = 475VA of overload) . I just want to make sure I am doing it the correct way.

 

[GoldDigger]

As was already said, the maximum fuse size is determined by the original KVA number using the nominal voltage, and does not change. Regardless of what the primary and secondary voltages are, the heating is based on the current squared times the winding resistance which is constant.
Reducing the applied voltage will reduce the idling copper and iron losses, but the effect of that on the maximum current handling will be small.

Sent from my XT1080 using Tapatalk

 

[Smart $]

If the KVA is Derated then it does not make much sense to use that KVA as the KVA for the fuse sizing. Yes, secondary is 95V. So using 3000 would have 31.57 secondary Amps. If I derate for the 380V I get 2.375KVA with secondary amps of 25A.

31.75 * 1.25 = 40A fuse

25 * 1.25 = 35A fuse.

Relatively big difference (5 * 95 = 475VA of overload) . I just want to make sure I am doing it the correct way.

Check your math...

3000VA ? 480V = 6.25A

3000VA ? (380V ? 480V) = 2375VA

2375VA ? 380V = 6.25A

Given the turns ratio is 1:4, your secondary rated current should be 25A regardless of the change in primary voltage.

 

[fifty60]

Excellent, thanks.

 

[fifty60]

Does a transformer experience its initial inrush regardless of the load attached to it?

I am trying to size a single phase 2KVA transformer that I want to use at 460 and also at 380VAC. The transformer has taps for 380 and 460, so it will produce 2KVA in either configuration.

For 460 I get 2000/460=4.34*1.67=7.26 so, a 7A OCPD.
For 380 I get 2000/380=5.26*1.67=8.79 so, an 8A ocpd

I want a on size OCPD fits all, for simplicity. I'm know there is only a 1A difference here. On my secondary I have 2 breakers, one 10A (for a 115V circuit) and the other 6A (for a 230V circuit).

For 2000VA/230V I get 10.86A, so I am over the combined requirement of OCPD's on the secondary to be able to move up to out of the 1.67 x's sizing for the primary.

I would like to use the 7A D-curve breakers for 380 and 460V. I'm just not confident that it will work well for both configurations, without nuisance trips at 380V.

 

[fifty60]

-(sorry duplicate) please see above for question

 

[kingpb]

The inrush of the transformer is independent of the load attached. It can vary depending on where the sine wave was when it was shut off. The fuses need to be able to ride thru the inrush, and also protect the transformer.

The NEC limits of transformer protection are very misleading. To accurately provide the protection you need the transformer damage curve and transformer data.

Sizing protection without those will not guarantee positive results.